photoproduction of positive pions from hydrogen with phoe ...€¦ · radiator foil and produces...

Nuclear Physics A570 (1994) 580-598 North-Holland

NUCLEAR PHYSICS A

Photoproduction of positive pions from hydrogen with PHOE~I~S at ELSA

K. Biichler, K.H. Althoff, G. Anton, J. At-ends, W. Beulertz, M. Breuer, P. DetempIe, H. Dutz, E. Kohlgarth, D. Kramer, W. Meyer, G. NSIdeke, W. Schneider, W. Thiel, B. Zucht

Physikalixhes Institut, Uniuersity of Bonn, Nussallee 1.2, O-53115 Bonn, Germany

Received 19 May 1993 (Revised 23 August 1993)

Abstract The differential cross section of the reaction yp + rr+n has been measured with the

PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies (E, = 220-900 MeV) and pion angles (@ii”‘.= 35”-135”) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.

Key words: NUCLEAR REACTIONS ‘H(-y, rr+ 1, E = 220-900 MeV bremsstrahlung; measured o(0). Model comparison

1. Introduction

Meson production from nucleons is one main approach to study the baryon resonance spectrum. The resonance parameters like mass, width and coupling strengths have been determined in an overall reasonable accuracy. But some important problems remained unsolved. In photon-induced reactions the most discussed examples in the first and second resonance region are the electric quadrupole strength of the yN + A(12321 transition or the electromagnetic excitation of the Roper resonance P&440).

The resonance coupling parameters are extracted from meson photoproduction data in the frame-work of partial-wave and multipole analyses. However, ambiguities in this procedure arise due to the incompleteness of the data sets for the polarization observables and due to the inconsistency of the data sets for the differential cross sections. Consequently, new experimental data of high precision

03759474/94/$07&l 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0375-9474(93)EO634-K

K Biichler et al. / Photoproduction of positive piom 581

are needed to improve the analyses. This high precision becomes accessible with the new generation of high duty factor electron accelerators, like ELSA in Bonn.

The PHOENICS experiment at ELSA [l] consists of a photon tagging spectrometer and a large-acceptance hadron detector. Accordingly, a process like yN + rrN can be measured over a large kinematic range simultaneously. Clean event recog- nition and background rejection is achieved by a considerable kinematic overdetermination. This experimental technique leads to a consistent data set in the first and second resonance region.

The measurement of the differential cross section of the reaction yp + rTT+n which is reported here is a first step of an intense study of the pion photoproduction with PHOENICS. Target asymmetry data are already measured and additional observables will be determined using a linear polarized photon beam combined with a polarized target.

2.

1.

Experimental set-up

A top view of the experimental set-up of PHOENICS at ELSA is shown in Fig. The electron beam extracted from ELSA has an energy of 1 GeV. It hits a

radiator foil and produces bremsstrahlung photons. The position and profile of the electron and the photon beam is measured by a beam monitor system [2]. The photon beam at the target has a circular profile with a diameter of 1.1 cm (FWHM).

FL

Electron Beam

\ ‘lhgglng Magnet

Beam Monitor \

‘ragging Hodoscope FR

Fig. 1. Top view on the experimental set-up of PHOENICS at ELSA.

K Biichler et al. / Photoproduction of positive pions

rad

photon beam c

recoil electron ( 400 MeV)

ron beam ( 1 GeV )

128 energy counters ( EC )

Yi \ 18 timing counters ( TC )

Fig. 2. Side view on the PHOENICS tagging system.

The energy and flux of the photons is determined by a tagging spectrometer [3]. It contains 128 small scintillation counters (see Fig. 2) in the focal plane of the spectrometer magnet leading to a photon energy resolution of 10 MeV (FWHM) for E, = 200-360 MeV and 5 MeV for E, = 360-950 MeV. 16 thick scintillation counters positioned behind the small counters are used for a time-of-flight measurement which yields the time of arrival of the correlated photon at the target.

The photon beam impacts on a liquid hydrogen target consisting of a cylindrical cell of 8 cm length (parallel to the beam) and 3 cm diameter. The outgoing particles are registered by the PHOENICS detector.

The PHOENICS detector is shown in more detail in Fig. 3. It consists of vertically oriented scintillator bars which are arranged in concentric ring segments around the target. Each scintillator bar is equipped with photomultiplier tubes on both vertical ends. The anode pulse of each phototube is split: one half is given to a charge integrating ADC (analog to digital converter), the other half has to pass a leading-edge discriminator and is then sent to a TDC.

The set-up is designed to detect 2 particles of the final state. One particle is registered with the counters left of the beam CL and FL (CL = close to the target, left of the beam; FL = far from the target, left of the beam), and the other particle with the counters right of the beam CR and FR. The left side, which contains thin scintillation counters only, can be used for the detection of charged particles. The

K Biichler et al. / Photoproduction of positive pions 583

Fig. 3. Schematic view of the PHOENICS detector.

counters FR on the right side have a thickness of 20 cm. So, this side can be used either for the detection of charged particles or for the detection of neutral particles like neutrons or photons.

For the measurement of yp + r+n the r+ was detected by the coincidence of the CL and FL counters while the neutron was detected by the FR counters requiring an anticoincidence with the CR counters. The neutrons occur under laboratory angles smaller than 80”. Therefore, the available FR counters were arranged in 2 rows in order to increase the neutron detection efficiency, see Fig. 3.

With the signals from the phototubes at both ends of a scintillator bar, the time-of-flight of a particle is given by the sum of the time measured by the top and bottom tubes. The vertical impact position is given by the difference of the time measured by these tubes. Each scintillator bar is positioned at a certain laboratory angle so that in combination with the information of the time-of-flight and the vertical impact position the three-component velocity vector (p,, p,,, p,) of a particle is measured (/3 = U/C). The precision of this information is directly related to the TOF resolution.

Table 1 Parameters of the scintillation counters of the PHOENICS detector. qime: time-of-flight resolution, uo; scattering angle resolution, a,: azimuthal angle resolution

Type Number Size (cm3) Distance to otime (PSI “o (‘1 o* (‘I thickness X width X height the target (cm)

CL 15 0.5x11x 51 70 210 4.0 2.3 CR 15 0.5x 11 x 51 70 210 4.0 2.3 FL 31 2 x20x170 300 250 1.8 0.8 FR 23 20 x 20 x 100 135 175 3.9 1.4

584 K. Biichler et al. / Photoproduction of positive piom

The time-of-flight resolution of the scintillators was determined in the electron test beam of the Bonn 2.5 GeV synchrotron. The results are listed in Table 1 together with typical laboratory angle resolutions.

In order to obtain a good time-of-flight information time calibration measurements were performed immediately before starting the yp -+ r+n measurement.

First, all TDC’s (time-to-digital converter) were calibrated concerning the time interval per channel with a time calibrator module.

For the relative time calibration of the tagging counters a plexiglass Cerenkov counter was positioned in the photon beam and coincidences of the Cerenkov with the tagger counters were recorded. The Cerenkov counter gave a time reference signal which enabled the alignment of the time-of-flight peaks of the TDC-spectra of the different tagger counters.

For the time calibration of the CL, FL, CR and FR counters e+e- pair production was employed. The efe- pairs, produced in the target and emitted under very small angles into forward direction, were deflected by a strong vertical magnetic field in the target area (produced by a magnet which is installed for a polarized target). Accordingly, a considerable amount of e+e- events could be recorded for all CL, FL, CR and FR counters. These events generate a pro- nounced “j3 = 1” peak in the TDC spectra.

The leading-edge discriminator causes a time walk of the produced output signal depending on the pulse height of the input signal. This time walk deterio- rates the TOF information measured by the TDC. The ADC information of “p = 1” particles was used to correct this time walk.

The 20 cm thick scintillation counters FR were used for the detection of the neutrons of the yp -+ r+n reaction. The neutron detection efficiency of four of these counters was measured at the SATURNE accelerator in Saclay (France).

energy threshold:

Q 1.1 MeV A 2.0 MeV

0 3.OMeV * 4.OMeV

0 25 50 75 100 125 150 175 200

neutron energy [MeV]

Fig. 4. Neutron counter efficiency as a function of the neutron energy [4].

K. Biichler et al. / Photoproduction of positive pions 585

The elastic np scattering reaction was measured, where the scattered proton was detected to tag the correlated neutron. By varying the elastic np scattering kinematic the neutron detection efficiency of our counters could be determined as a function of the neutron energy, see Fig. 4. Furthermore, the dependence of the efficiency on the neutron impact position on the counter was investigated [4].

It can be seen from Fig. 4 that the neutron detection efficiency depends strongly on the energy threshold defined by the discriminator threshold. This energy threshold was calibrated with the signal from a ‘jaCo y-source and cosmic radia- tion. It was monitored with a LED-monitor system.

3. Measurement and data analysis

The data for the differential cross section of the yp + rTT+n reaction were taken during two runs in a total time of 150 hours. The duty factor of the extracted beam was 35-40%, the tagged photon intensity was NY = 1 X lo6 s-l. A total number of 1.2 million events was recorded, out of which about 20% were identified as yp + r+n events.

In Fig. 5 the velocity (p = v/c> of the charged particle (measured by the CL and FL counters) is plotted versus the velocity of the neutral particle (measured by the FR counters). Three types of events can be seen in this plot:

(i) r+n events are located at a low velocity of the neutral particle (neutron) and high velocity of the charged particle (~‘1.

(ii) rap events are detected via one or two of the decay photons of the r” on the neutral particle side, i.e. high velocity on the neutral-particle side (photon) and low velocity on the charged-particle side (proton).

(iii) e+e- events occur with high velocity of both particles. In addition they occur as random coincidences between the tagger (which gives the TOF start) and the PHOENICS-detector. Accordingly, they appear as events with two particles of the same (but arbitrary) velocity.

Multi-pion events and accidentals give background contributions. In contradic- tion to the trigger condition, e+e- events are contained in the data. This is due to

the fact that the geometrical size and position of the CR counters did not perfectly match the vertical size and position of the FR counters; i.e. there was a small “veto leakage” in the set-up. It can be seen from the kinematical plots of Fig. 6 that this leakage did not hinder the a+n event identification.

The rap events were not further analyzed, because the detector acceptance for the yp + rap reaction has not been investigated. The aim of the PHOENICS experiment is mainly the measurement of polarization observables. In that case the knowledge of the detector acceptance is not necessary and consequently polarization observables of yp + r+n and yp + rap can be determined simultaneously.

586 K. Biichler et al. / Photoproduction of positive pions

lk corrected data

time walk corrected data

Fig. 5. Velocity (p = u/c) measured for the charged particle by the coincidence of the CL and FL

counters plotted versus the velocity of the neutral particle measured by the FR and the anticoincidence

with the CR counters. The start of the time-of-flight measurement was given by the tagger counters.

Top left: raw data covering the total energy and angular range. Top right: the same data after time-walk

correction; the line indicates a cut on rr+ n events. Bottom: the same as top right.

The experimental set-up delivers the following seven kinematical quantities for each event of the type photon + proton + “particle l”+ “particle 2” + anything: -Energy E,, of the photon. -TOF,, O,, Q1 of particle “1”. -TOF,, O,, QZ of particle “2”.

Each event is analysed under the assumption of a yp --) r+n event. Accord- ingly, two kinematical parameters, for example the photon energy and the pion scattering angle, are sufficient to determine the kinematics. As seven kinematical parameters are measured, the overdetermined parameters are used to test the assumption. The expected value of each parameter can be compared with the measured value event by event, e.g. the velocity of the particle detected by the CL and FL counters via the time-of-flight information. This measured velocity p,,,, is compared to Pcalc, which is the velocity of the pion calculated from the yp + rTT+n kinematic for that event. This comparison was done for the following quantities: velocity of the pion, velocity of the neutron, scattering angle of the neutron and azimuthal angle of the pion compared to the neutron (coplanarity), see Fig. 6.

K. Bkhler et al. / Photoproduction of positive pions

_~~~~

7

~~~~~

-1 -0.5 0 0.5 -1 -0.5 0 0.5 1

587

Fig. 6. Distribution of kinematical obsetvables. 9, + @,: Sum of the azimuthal angles of the detected particle “1” and particle “2”; coplanar events like pf n should give @t + @, = 0. Oylc - BP: Measured polar angle of particle “2” (i.e. neutron of w+ n) minus calculated polar angle of particle “2” assuming the kinematics yp + nf n: pylc - py: Measured velocity of particle “2” minus calculated velocity of particle “2”: pFalC - py: Measured velocity of particle “1” minus calculated velocity of particle “1”. Dotted curve: including all data, full curve: after a soft cut on the “n+n” like events

according to Fig. 5.

Obviously, these plots show a clear peak at “0” visualizing the yp -+ rTT+n events. Combining all the 4 informations in a kinematical fit the kinematic overdetermination leads to a very good suppression of background reactions.

After the subtraction of background and of empty target contributions the resulting event number was used for the determination of the differential cross

Table 2 Correction contributions to the count rate and the resulting contribution to the error of the differential cross section. The error of the neutron counter efficiency varies depending on the neutron energy

Correction Resulting error for the diff. cross section

background l-4% < 1% empty target < 1% < 1% neutron counter efficiency 0.15-0.22 2.5-6.5% photon definition probability 0.73-0.84 0.8-2.3% other contributions < 3%

588 K Biichler et al. / Photoproduction of positive pions

Table 3 Results of the differential cross section measurement yp * rr+n

0 C.M.S.

(deg)

W c.m.s. (MeV)

du,‘df2

Wsr)

Stat. error

(pb/sr)

Syst. error

(%o)

247 44.5 1159.2 6.91 0.353 6.9 247 53.6 1159.2 9.79 0.360 6.0 247 62.9 1159.2 12.12 0.423 5.4 247 71.9 1159.2 13.57 0.439 5.1 247 80.8 1159.2 14.25 0.438 5.1 247 88.8 1159.2 14.31 0.457 4.9 247 97.0 1159.2 15.19 0.470 4.8 247 104.8 1159.2 15.34 0.497 4.6 247 112.2 1159.2 15.83 0.526 4.4 247 119.4 1159.2 17.05 0.571 4.3 247 126.3 1159.2 15.53 0.584 4.2 247 133.0 1159.2 11.2s 0.532 4.2 298 44.8 1199.8 11.38 0.398 6.8 298 54.4 1199.8 15.93 0.502 6.0 298 63.8 1199.8 18.12 0.528 5.3 298 72.5 1199.8 19.64 0.577 4.6 298 81.2 1199.8 21.16 0.577 4.2 298 89.5 1199.8 22.04 0.631 4.2 298 97.5 1199.8 22.50 0.672 4.1 298 105.1 1199.8 23.79 0.721 4.2 298 112.4 1199.8 22.80 0.778 4.2 298 119.8 1199.8 23.60 0.832 4.1 298 126.5 1199.8 21.89 0.839 4.1 351 36.1 1240.6 11.80 0.461 7.0 351 45.8 1240.6 14.77 0.494 6.2 351 55.1 1240.6 15.40 0.493 5.5 351 64.9 1240.6 16.35 0.516 4.9 351 73.2 1240.6 16.26 0.574 4.4 351 82.3 1240.6 17.03 0.623 4.1 351 90.3 1240.6 16.64 0.638 4.1 351 98.4 1240.6 15.79 0.671 4.1 351 106.0 1240.6 15.17 0.692 4.2 351 113.5 1240.6 14.85 0.741 4.1 351 120.7 1240.6 14.89 0.776 4.1 351 127.2 1240.6 13.84 0.794 4.2 351 133.6 1240.6 12.83 0.835 4.2 399 36.4 1276.4 12.90 0.576 6.4 399 46.6 1276.4 11.98 0.463 6.0 399 56.7 1276.4 11.66 0.460 5.5 399 65.8 1276.4 11.71 0.497 5.3 399 74.8 1276.4 10.54 0.53s 4.9 399 83.4 1276.4 10.30 0.565 4.7 399 91.9 1276.4 9.86 0.580 4.7 399 100.0 1276.4 8.86 0.590 4.7 399 106.9 1276.4 9.04 0.618 4.8 399 114.7 1276.4 8.21 0.640 4.7

K Bikhler et al. / Photoproduction of positive pions 589

Table 3 (continued)

E, (MeV)

8 c.m.s.

(deg)

W c.m.s.

(MeV)

du/dfI

(pb/sr)

Stat. error

W/w) Syst. error

(%I

399 121.4 1276.4 7.77 0.639 4.8 399 128.3 1276.4 6.44 0.613 4.8 399 134.9 1276.4 5.08 0.570 4.8 452 31.3 1314.7 11.14 0.559 6.2 452 41.8 1314.7 10.74 0.462 6.0 452 57.8 1314.7 10.14 0.472 5.7 452 66.9 1314.7 10.25 0.541 5.5 452 16.3 1314.7 9.43 0587 5.3 452 84.6 1314.7 8.33 0.591 5.0 452 93.5 1314.7 7.02 0.584 5.1 452 101.3 1314.7 6.35 0.584 5.0 452 108.9 1314.7 6.13 0.587 5.0 452 115.8 1314.7 5.45 0.596 5.1 452 122.7 1314.7 5.38 0.604 5.1 452 129.6 1314.7 4.31 0.632 5.0 452 135.8 1314.7 3.68 0.583 5.0 501 31.9 1349.3 9.99 0.501 5.4 501 48.7 1349.3 9.38 0.434 5.3 501 58.5 1349.3 8.62 0.456 5.1 501 68.5 1349.3 9.53 0.555 5.1 501 71.2 1349.3 8.33 0.588 5.0 501 86.3 1349.3 6.64 0.546 5.0 501 94.2 1349.3 5.59 0.518 5.1 501 102.4 1349.3 5.19 0.533 5.0 501 110.0 1349.3 4.74 0.532 5.0 501 116.9 1349.3 3.57 0.506 5.0 501 123.8 1349.3 3.65 0.528 5.1 501 130.5 1349.3 2.45 0.462 5.0 501 136.8 1349.3 1.97 0.447 5.0 551 39.0 1383.6 10.75 0.607 6.1 551 49.6 1383.6 10.07 0.551 5.9 551 59.6 1383.6 9.63 0.611 5.6 551 70.0 1383.6 10.00 0.686 5.5 551 78.8 1383.6 8.33 0.723 5.3 551 87.2 1383.6 6.91 0.659 5.2 551 95.9 1383.6 5.34 0.650 5.0 551 103.3 1383.6 4.96 0.644 5.0 551 111.1 1383.6 4.76 0.660 5.1 551 118.2 1383.6 3.79 0.696 5.0 551 124.9 1383.6 3.73 0.642 5.0 551 131.4 1383.6 2.56 0.642 5.1 600 39.5 1416.4 10.02 0.555 6.3 600 50.7 1416.4 10.15 0.565 6.1 600 60.7 1416.4 9.85 0.629 5.8 600 70.9 1416.4 9.68 0.719 5.6 600 80.3 1416.4 7.25 0.703 5.4 600 88.8 1416.4 6.80 0.675 5.3 600 97.3 1416.4 5.70 0.687 5.3


Table 3 (continued)

E? (MeV)

0 c.m.s.

(deg)

W C.rn.S. (MeV)

da/dR

&b/x)

Stat. error

&b/w)

Syst. error

(%o)

600 105.3 1416.4 5.26 0.668 5.2 600 111.8 1416.4 4.08 0.633 5.3 600 119.3 1416.4 3.73 0.653 5.3 600 126.2 1416.4 2.86 0.636 5.4 600 132.5 1416.4 2.22 0.660 5.3 650 40.2 1449.2 11.08 0.618 5.4 650 51.6 1449.2 11.09 0.630 5.4 650 61.6 1449.2 10.85 0.686 5.4 6.50 71.8 1449.2 9.95 0.751 5.3 650 81.2 1449.2 8.12 0.730 5.3 6.50 90.4 1449.2 7.03 0.708 5.3 650 98.1 1449.2 5.48 0.711 5.2 650 106.4 1449.2 5.01 0.676 5.3 650 113.5 1449.2 3.89 0.666 5.3 650 120.9 1449.2 3.36 0.655 5.3 650 127.1 1449.2 2.72 0.620 5.2 650 133.0 1449.2 2.27 0.716 5.3 103 41.1 1483.1 12.24 0.720 5.4 703 52.7 1483.1 12.60 0.733 5.4 703 62.7 1483.1 10.96 0.764 5.4 703 73.0 1483.1 11.30 0.863 5.4 703 82.3 1483.1 8.47 0.832 5.3 703 91.2 1483.1 8.15 0.823 5.4 703 99.1 1483.1 6.61 0.815 5.3 703 107.3 1483.1 6.11 0.805 5.3 703 114.7 1483.1 4.36 0.770 5.3 703 121.4 1483.1 3.95 0.752 5.3 703 128.0 1483.1 2.74 0.764 5.3 703 134.1 1483.1 2.44 0.844 5.3 751 42.1 1513.2 10.09 0.655 5.6 751 53.2 1513.2 9.65 0.664 5.6 751 63.8 1513.2 9.82 0.736 5.6 751 13.7 1513.2 8.61 0.779 5.5 751 83.9 1513.2 8.70 0.850 5.6 751 92.3 1513.2 7.46 0.829 5.5 751 100.4 1513.2 6.89 0.830 5.5 751 108.4 1513.2 5.41 0.819 5.5 751 115.9 1513.2 4.93 0.836 5.5 751 122.6 1513.2 3.52 0.830 5.4 751 128.8 1513.2 2.81 0.873 5.5 800 43.0 1543.2 7.46 0.643 5.6 800 54.5 1543.2 6.38 0.596 5.6 800 64.9 1543.2 5.35 0.607 5.6 800 74.9 1543.2 5.37 0.689 5.5 800 84.8 1543.2 3.89 0.623 5.5 800 93.4 1543.2 4.35 0.698 5.5 800 102.1 1543.2 4.52 0.805 5.5 800 109.3 1543.2 4.68 0.853 5.5

Table 3 (continued)


E, (MeV)

0 c.m.s. (deg)

du/dfJ

&b/u)

Stat. error

&b/x)

Syst. error

(%)

800 116.8 1543.2 800 123.3 1543.2 800 129.7 1543.2 852 43.7 1574.5 852 55.2 1574.5 852 66.0 1574.5 852 76.5 1574.5 852 85.8 1574.5 852 94.8 1574.5 852 102.7 1574.5 852 110.4 1574.5 852 117.7 1574.5 852 124.4 1574.5 852 130.6 1574.5 900 44.3 1602.9 900 56.3 1602.9 900 67.2 1602.9 900 77.4 1602.9 900 86.6 1602.9 900 95.7 1602.9 900 103.9 1602.9 900 111.3 1602.9 900 118.6 1602.9 900 125.1 1602.9 220 45 1137.2 241 45 1154.4 261 45 1170.5 282 45 1187.2 303 45 1203.7 325 45 1220.7 345 45 1236.0 366 45 1251.9 388 45 1268.2 409 45 1283.7 452 45 1314.7 501 45 1349.3 551 45 1383.6 600 45 1416.4 650 45 1449.2 703 45 1483.1 751 45 1513.2 800 45 1543.2 852 45 1574.5 900 45 1602.9 220 60 1137.2 241 60 1154.4 261 60 1170.5 282 60 1187.2

4.04 0.949 5.4 3.92 1.008 5.4 3.61 1.172 5.5 6.21 0.544 5.7 5.23 0.508 5.7 3.93 0.486 5.7 3.05 0.478 5.7 2.48 0.471 5.6 2.68 0.517 5.6 2.59 0.560 5.6 2.65 0.612 5.6 2.11 0.613 5.6 1.97 0.841 5.6 2.05 0.806 5.6 6.96 0.647 5.8 5.07 0.551 5.8 4.05 0.556 5.8 2.14 0.431 5.8 1.97 0.452 5.7 2.47 0.563 5.7 2.37 0.575 5.7 2.36 0.645 5.7 1.74 0.765 5.7 1.84 0.689 5.7

6.77 0.37 6.9 8.26 0.40 6.8

10.58 0.50 6.8 12.84 0.51 6.8 14.54 0.58 6.6 14.73 0.61 6.4 14.64 0.62 6.1 12.89 0.61 6.0 11.73 0.63 6.0 10.74 0.46 6.0 9.58 0.48 5.3

10.26 0.58 5.9 10.67 0.56 6.1 11.09 0.62 5.4 12.24 0.72 5.4 9.98 0.66 5.6 7.35 0.63 5.6 6.21 0.54 5.7 6.76 0.65 5.8 8.10 0.40 5.4

11.03 0.48 5.3 12.31 0.52 5.1 15.54 0.62 4.8

592

Table 3 (continued)

K Biichler et al. / Photoproduction of positive piom

0 W C.rn.S. C.rn.S. (deg) (MeV)

da/d0 Stat. error

(pb/sr) kb/sr)

Syst. error

(%)

303 60 325 60 345 60 366 60 388 60 409 60 452 60 501 60 551 60 600 60 650 60 703 60 751 60 800 60 852 60 900 60 220 90 241 90 261 90 282 90 303 90 325 90 345 90 366 90 388 90 409 90 452 90 501 90 551 90 600 90 650 90 703 90 751 90 800 90 852 90 900 90 220 120 241 120 261 120 282 120 303 120 325 120 345 120 366 120 388 120 409 120 452 120 501 120

1203.7 17.88 0.64 4.7 1220.7 18.16 0.65 4.8 1236.0 16.53 0.63 4.9 1251.9 14.83 0.60 5.0 1268.2 12.78 0.59 5.2 1283.7 10.09 0.55 5.2 1314.7 10.15 0.49 5.1 1349.3 8.62 0.47 5.2 1383.6 9.63 0.61 5.2 1416.4 9.86 0.63 5.3 1449.2 10.85 0.69 5.3 1483.1 11.26 0.76 5.3 1513.2 9.81 0.74 5.4 1543.2 5.84 0.60 5.4 1574.5 4.63 0.49 5.5 1602.9 4.66 0.55 5.5 1137.2 11.41 0.49 5.1 1154.4 13.49 0.53 4.9 1170.5 15.87 0.58 4.5 1187.2 18.69 0.68 4.3 1203.7 22.61 0.79 4.2 1220.7 19.69 0.77 4.2 1236.0 17.61 0.80 4.1 1251.9 14.48 0.76 4.3 1268.2 11.09 0.73 4.5 1283.7 9.22 0.71 4.7 1314.7 7.82 0.59 5.0 1349.3 6.24 0.54 5.0 1383.6 6.81 0.66 5.0 1416.4 6.80 0.68 5.3 1449.2 7.03 0.71 5.3 1483.1 8.15 0.82 5.4 1513.2 7.47 0.83 5.5 1543.2 4.36 0.69 5.5 1574.5 2.58 0.49 5.6 1602.9 2.18 0.51 5.7 1137.2 11.23 0.54 4.2 1154.4 16.24 0.67 4.2 1170.5 18.20 0.75 4.2 1187.2 22.35 0.93 4.1 1203.7 24.09 1.05 4.3 1220.7 21.01 1.04 4.4 1236.0 16.22 0.98 4.4 1251.9 13.61 0.95 4.6 1268.2 9.12 0.82 4.7 1283.7 6.26 0.73 4.8 1314.7 5.43 0.60 5.0 1349.3 3.60 0.51 5.0


Table 3 (continued)

E, Q,.,.,. c.m.s. W do/d0 Stat. error Syst. error

(MeV) (deg) (MeV) (wb/sr) (pb/sr) (%I

551 120 1383.6 3.73 0.70 5.0 600 120 1416.4 3.64 0.65 5.2 650 120 1449.2 3.37 0.66 5.3 703 120 1483.1 4.04 0.75 5.3 751 120 1513.2 4.07 0.83 5.5 800 120 1543.2 3.92 0.89 5.5 852 120 1574.5 2.11 0.61 5.6 900 120 1602.9 1.74 0.76 5.7

section of yp --f rTT+n. The acceptance of the detector was obtained by a Monte Carlo simulation. Furthermore, corrections due to loss of pions (decay) and nuclear absorption were applied. For the tagger, only single hits of the 128 energy defining counters were accepted with the exception of double hits of neighbouring counters. The related reduction of the photon flw was taken into account.

The main contributions to the error of the differential cross section are listed in Table 2. The largest contribution is due to the uncertainty of the neutron detection efficiency. It should be noticed that the relative error due to the background subtraction was smaller than 1% for all kinematical points. In the mean time the handling of the tagging system has been improved so that the error of the photon definition probability and the photon flux correction will be smaller for further measurements. In addition, the duty factor of ELSA has been increased to about 60%.

4. Results and discussion

The differential cross section of the reaction yp + r+n was measured for photon energies E,, = 220-900 MeV and pion scattering angles of f?J:“‘.= 35”-135”. The energy-defining counters of the tagging system were grouped together so that photon energy bins of AE, = f 15 MeV were obtained. The differential cross section of yp + rr+n was determined for angular bins of AOzb = f3.8”. The results are listed in Table 3 and plotted in Fig. 7.

In Fig. 8 excitation curves for four different angles are plotted together with older data. The agreement with the Orsay data 161 and with the former Bonn data [7] is good. The Tokyo data [8], especially at small angles, show considerable deviations.

A comparison of our data with the multipole analysis of Arndt et al. 191 is shown in Fig. 9. The curve represents the fit of the analysis already including our data. The chi-squared per data point is 1.5 before our data are included in the fit [lo].


r 30

z! _ Ey=(246f15)MeV _ Ey=(298*15)MeV _ Ey=(351*15)MeV

9 *4+ 2 20 -

.*' * .

..**. . * . 0. 00'0. l ** .**

. . . . % .

.

18 I I I I I I I I I I I I I I _ Ey= (399 If: 15) MeV _ Ey= (452 + 15) MeV _ Ey= (501 f 15) MeV

12 - 'w %

+'+, - "**

'+ '4 +

'+ 6- +

+ '++ +%

*++, *+ 'r

16 I I I I I 1 I I I I I I I I - Ey= (55lk 15) MeV - Ey= (600 f 15) MeV - Ey= (650 f 15) MeV

10 - ++++ _ ++++

++

- ++++

++ ++ 5- +++

+++

++ ++

++

++ ++++

16 I I I I I I I I I I I I I I I - Ey= (703 f 15) MeV - Ey= (751 f 15) MeV - Ey= (800 + 15) MeV

12 - ++++

++ +++++

6- ++ ++ ++

- ++

++ ++ ++

y++++++

1Q I I I I I I I I I I I I I I

- Ey=(852+15)MeV - Ey=(9OOflS)MeV () 90 180

8-

+* +

4- ++++++

++

+++ +++++++

0 I I I I I I I 1 I

0 90 180 90 180

%ms [degrees]

Fig. 7. Measured differential cross section of yp + CT+ n.


6

4

26: . ..‘.~..‘...~‘.~.‘~..~‘~..~‘~~.~‘..- n 82670RsA - a B267OnsA

24

Fig. 8. Comparison with data from other laboratories.

14

12

10

a

6

4

2

0

12

10 Ey= (!300 f 15) MeV

a

6

4

2

0 0 90 la0 0

8 n,cMs kve4 8 n,CMS kwee4

Fig. 9. Differential cross section of yp -+ T+ n compared to the result of the partial wave analysis of Arndt et al. 191.

596 I2 Biichler et al. / Photoproduction of positive pions

T 30

J? Ey = (298 f 15) MeV

0 1 / I t 0 90 1 IO

0 n,CMS bw-efd

Fig. IO. Differential cross section of yp -+ v+n compared to the dynamical model of Nozawa et al. Ill].

This good value reduces slightly down to 1.2 when our data are included in the fit. This result is directly related to a good consistency of the data.

A dynamical model of pion photoproduction which ensures gauge invariance and unitarity was developed by Nozawa, Blankleider and Lee ill]. It takes into account the channel space of TN, yN and the baryon states N and d. A

T 28

2 24

% $j

:: 12

8

4

0 0 90 180

18 16 14 12 10 8 6 4 2 0 I I

0 90 180

I\ Ey=@OOf15)MeV

0 90 180 0 90 180

Fig. 11. Differential cross section of yp -+ T+ n compared to the phenomenological lagrangian model of Carcilazo et al. [12].


comparison with our data in the delta resonance region is shown in Fig. 10. The agreement is rather good.

A phenomenological lagrangian model was performed by Garcilazo and Moya de Guerra [12]. This model contains the P33 in the first and the Sii, Pi1, Di3, S,, and D,, in the second resonance region. Also for this model the agreement with our data is good, see Fig. 11. Only for the highest photon energy of 900 MeV the shape of the angular distribution is considerably different.

5. Conclusion

In spite of a large number of existing data for the pion photoproduction on nucleons, the theoretical interpretation is not free of ambiguities. This is mainly due to systematic inconsistencies of the data, obtained in different experiments for different kinematical regions.

The presented tagged-photon experiment allowed for the first time a simultane- ous measurement over a large range of photon energies (220-900 MeV) and pion c.m.-angles (35”-135”). Due to the considerable kinematic overdetermination a very clean event signature could be achieved. Accordingly, the data contain a small and well-known systematic error.

The contribution of small amplitudes can only be determined if polarization observables are measured. Therefore, the PHOENICS detector has been used to measure the target asymmetry of the reaction yp + r+n and yp + rap in the same kinematical range. The data are under evaluation. Further experiments to determine additional polarization observables with linear polarized photons and a polarized target are in preparation.

We would like to thank the workshops of the Physikalisches Institut for the manufacturing of components and the accelerator crew under Prof. D. Husmann for the excellent running of EISA. We also like to thank the group of J.P. Didelez and the accelerator crew of the “Laboratoire Nationale Satume” for the substan- tial help during the neutron counter efficiency measurement.

This work was supported by the BMFT.

References

[l] D. Husmann et al., ELSA - the continuous beam accelerator at Bonn, Proc. european particle accelerator Conf., Rome, June 7-11, 1988

[2] J. Arends et al., Nucl. Instr. Meth. A306 (1992) 89 [3] P. Detemple et al., Nucl. Instr. Meth. A 321 (1992) 479 [4] N. Harpes, diploma thesis, Bonn-IR-91-50 (1991)


[5] K. Buechler, PhD thesis, Bonn-IR-91-57 (1991)

[6] C. Betourne et al., Phys. Rev. 172 (1968) 1343

[7] G. Fischer et al., Z. Phys. 253 (1972) 38

[8] T. Fujii et al., Nucl. Phys. B 120 (1977) 395

[9] R.A. Arndt, R.L. Workman, Z. Li and L.D. Roper, Phys. Rev. C42 (1990) 1853

[lo] R.L. Workman, private communication

[ll] S. Nozawa, B. Blankleider and T.S. Lee, Nucl. Phys. A 513 (1990) 459

[12] H. Garcilazo and E. Moya de Guerra, to appear in Nucl. Phys. A

photoproduction of positive pions from hydrogen with phoe ...€¦ · radiator foil and produces...

Documents